This invention relates to corrosion prevention in water heaters, and more particularly to improved cathodic protection systems which supplement glass coatings in protecting steel tanks. Specifically, it defines easily applied coatings for sacrificial anodes to improve their performance, which coatings can also be applied to exposed metal in the tank and electric heating elements to reduce the amount of protection they absorb.
In prior art, sacrificial anodes of active metals like magnesium and aluminum are suspended in water heater tanks to provide cathodic protection for steel not covered by glass coatings. Such exposed areas include weld seams, protruding spuds, and defects in the glass itself. Because of their reactivity, these anodes are attacked faster by hot water than exposed steel. Electrons produced are transferred to the tank wall where they polarize exposed areas and prevent their corrosion. Protection is said to be cathodic because the anode forms an electrochemical cell with the tank wall as cathode, and preferentially corrodes to protect exposed areas of steel there. Anode usage is proportional to the amount of exposed steel present. Since this is small, they remain active for many years, but many other factors also increase anode usage. Premature loss of this cathodic protection results in accelerated corrosion of steel at these exposed areas causing failure of the tank before its warranted life.
Ideally, sacrificial anodes dissolve uniformly over their length, converting all of their active metal to cathodic protection. However, in certain waters, when large amounts of exposed steel are present in the tank head, anodes are preferentially attacked at their top end near the tank connection because of the closeness of this exposed steel and metal of fittings. This causes "necking down" there which can sever the tank connection before the active metal is completely used. The fitting holding the anode affects necking most because it is closest to the active anode metal. This problem is most severe in low conductivity water where it is difficult to draw protection from the end of the anode furthest from this connection for the large amount of exposed steel in the tank head.
Another problem of conventional anodes is their excessive usage in certain high conductivity waters. Here high levels of dissolved salts accelerate hot water attack of the anode metal, producing more electrons than needed for corrosion protection. These dissolved salts also facilitate interaction of anode electrons with surrounding water to form hydrogen gas, rather than being transferred to the tank wall for corrosion protection. Both these effects cause premature exhaustion of the anode. They also reduce its electrochemical efficiency which is the amount of usable protection produced per gram of active metal, so that larger anodes are needed for a given tank life.
Efficiency of anode metals is also reduced by formation of inert surface films. These impermeable barriers prevent interaction of water and active metal at certain areas of the anode, reducing the rate that protection is produced. They can even totally deactivate the anode. Such inert films are formed by accumulation of reaction products such as insoluble oxides on the anode. In other cases films are formed which reduce the electrochemical activity of the anode below that of exposed steel, so that the tank wall preferentially corrodes to protect the anode film (reversals). A related problem is the low efficiency of anodes in electric heaters caused by the proximity of highly charged, heating elements whose strong electronic fields draw off most of their electrons, shortening anode life and reducing the protection available for the rest of the tank.
Other problems with conventional anodes are the objectionable nature of their byproducts. Bad odors are produced in some sulfur containing waters when the anode interacts with anaerobic bacteria formed during heater operation. The more active the anode metal, the greater the possibility that odors will form. Unfortunately these more active metals provide the best cathodic protection. Anode operation also adds trace amounts of heavy metal ions to the tank water. When anode usage is high, such as in aggressive waters, these additions become more significant, and could have possible toxicological effects. For example, aluminum ions have tentatively been linked to Alzheimer's disease. Reducing this type of contamination could become a greater issue as stricter regulations on water purity are enacted.